linux-kernel - Re: [PATCH] tty: rp2: Fix reset with non forgiving PCIe host bridges

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Message-ID: <fb68e9ab-06b1-43b9-906a-9c1e83bee2c7@broadcom.com>
Date: Wed, 11 Sep 2024 17:19:08 -0700
From: Florian Fainelli <florian.fainelli@...adcom.com>
To: Jim Quinlan <james.quinlan@...adcom.com>
Cc: linux-serial@...r.kernel.org, Kevin Cernekee <cernekee@...il.com>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 Jiri Slaby <jirislaby@...nel.org>, John Ogness <john.ogness@...utronix.de>,
 Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>,
 Thomas Gleixner <tglx@...utronix.de>,
 "open list:TTY LAYER AND SERIAL DRIVERS" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] tty: rp2: Fix reset with non forgiving PCIe host bridges

On 9/11/24 15:44, Jim Quinlan wrote:
> On Wed, Sep 11, 2024 at 6:19 PM Florian Fainelli
> <florian.fainelli@...adcom.com> wrote:
>>
>> On 9/11/24 15:16, Jim Quinlan wrote:
>>> On Wed, Sep 11, 2024 at 6:01 PM Florian Fainelli
>>> <florian.fainelli@...adcom.com> wrote:
>>>>
>>>> On 9/11/24 14:47, Jim Quinlan wrote:
>>>>> On Fri, Sep 6, 2024 at 6:54 PM Florian Fainelli
>>>>> <florian.fainelli@...adcom.com> wrote:
>>>>>>
>>>>>> The write to RP2_GLOBAL_CMD followed by an immediate read of
>>>>>> RP2_GLOBAL_CMD in rp2_reset_asic() is intented to flush out the write,
>>>>>> however by then the device is already in reset and cannot respond to a
>>>>>> memory cycle access.
>>>>>>
>>>>>> On platforms such as the Raspberry Pi 4 and others using the
>>>>>> pcie-brcmstb.c driver, any memory access to a device that cannot respond
>>>>>> is met with a fatal system error, rather than being substituted with all
>>>>>> 1s as is usually the case on PC platforms.
>>>>>>
>>>>>> Swapping the delay and the read ensures that the device has finished
>>>>>> resetting before we attempt to read from it.
>>>>>>
>>>>>> Fixes: 7d9f49afa451 ("serial: rp2: New driver for Comtrol RocketPort 2 cards")
>>>>>> Suggested-by: Jim Quinlan <james.quinlan@...adcom.com>
>>>>>> Signed-off-by: Florian Fainelli <florian.fainelli@...adcom.com>
>>>>>> ---
>>>>>>     drivers/tty/serial/rp2.c | 2 +-
>>>>>>     1 file changed, 1 insertion(+), 1 deletion(-)
>>>>>>
>>>>>> diff --git a/drivers/tty/serial/rp2.c b/drivers/tty/serial/rp2.c
>>>>>> index 4132fcff7d4e..8bab2aedc499 100644
>>>>>> --- a/drivers/tty/serial/rp2.c
>>>>>> +++ b/drivers/tty/serial/rp2.c
>>>>>> @@ -577,8 +577,8 @@ static void rp2_reset_asic(struct rp2_card *card, unsigned int asic_id)
>>>>>>            u32 clk_cfg;
>>>>>>
>>>>>>            writew(1, base + RP2_GLOBAL_CMD);
>>>>>> -       readw(base + RP2_GLOBAL_CMD);
>>>>>>            msleep(100);
>>>>>> +       readw(base + RP2_GLOBAL_CMD);
>>>>>
>>>>> Since the assumed purpose of the readw() was to flush the writew(),
>>>>> would it make sense to add a barrier after the writew()?
>>>>
>>>> AFAICT there is one which is implied within the name, as it is not a
>>>> _relaxed() variant. Did you mean a different sort of barrier to be used?
>>>
>>> Not sure.  The __raw_writew() is followed by __io_aw(), which is a
>>> no-op on arm64.  I don't know arm64 well enough to know if a follow-up
>>> barrier is needed.
>>
>> By definition all of the {read,write}{b,w,l,q} do include an adequate
>> barrier
> I do see this in the kernel, e.g. altera_edac.c, pci-hyperv.c,
> oxu210hp-hcd.c, etc:
> 
>          write[lw](..);
>          wmb();
> 
> All I am saying is that the definition of writew() for arm64 has no
> explicit barrier *after* it makes the __raw_writew() call, since its
> __io_aw() call is a no-op.  I really don't know if this matters, just
> wanted to mention it.

Not having the documentation for this peripheral, not sure TBH.

As far as the drivers you quoted given the __io_bw() does include a 
barrier, it would appear that the barrier after is possibly redundant, 
or is based upon a misunderstanding of which side of the write the 
barrier must be on, or maybe they are actually necessary, but 
undocumented as such..
-- 
Florian